﻿/*
Derived from
Lengyel, Eric. "Computing Tangent Space Basis Vectors for an Arbitrary Mesh". Terathon Software 3D Graphics Library, 2001.
http://www.terathon.com/code/tangent.html
*/

using UnityEngine;
using System.Collections;

public static class TangentSolver
{

    public static void Solve(Mesh theMesh)
    {

        int vertexCount = theMesh.vertexCount;
        Vector3[] vertices = theMesh.vertices;
        Vector3[] normals = theMesh.normals;
        Vector2[] texcoords = theMesh.uv;
        int[] triangles = theMesh.triangles;
        int triangleCount = triangles.Length / 3;

        Vector4[] tangents = new Vector4[vertexCount];
        Vector3[] tan1 = new Vector3[vertexCount];
        Vector3[] tan2 = new Vector3[vertexCount];

        int tri = 0;

        for (int i = 0; i < (triangleCount); i++)
        {

            int i1 = triangles[tri];
            int i2 = triangles[tri + 1];
            int i3 = triangles[tri + 2];

            Vector3 v1 = vertices[i1];
            Vector3 v2 = vertices[i2];
            Vector3 v3 = vertices[i3];

            Vector2 w1 = texcoords[i1];
            Vector2 w2 = texcoords[i2];
            Vector2 w3 = texcoords[i3];

            float x1 = v2.x - v1.x;
            float x2 = v3.x - v1.x;
            float y1 = v2.y - v1.y;
            float y2 = v3.y - v1.y;
            float z1 = v2.z - v1.z;
            float z2 = v3.z - v1.z;

            float s1 = w2.x - w1.x;
            float s2 = w3.x - w1.x;
            float t1 = w2.y - w1.y;
            float t2 = w3.y - w1.y;

            float r = 1.0f / (s1 * t2 - s2 * t1);
            Vector3 sdir = new Vector3((t2 * x1 - t1 * x2) * r, (t2 * y1 - t1 * y2) * r, (t2 * z1 - t1 * z2) * r);
            Vector3 tdir = new Vector3((s1 * x2 - s2 * x1) * r, (s1 * y2 - s2 * y1) * r, (s1 * z2 - s2 * z1) * r);

            tan1[i1] += sdir;
            tan1[i2] += sdir;
            tan1[i3] += sdir;

            tan2[i1] += tdir;
            tan2[i2] += tdir;
            tan2[i3] += tdir;

            tri += 3;

        }



        for (int i = 0; i < (vertexCount); i++)
        {

            Vector3 n = normals[i];
            Vector3 t = tan1[i];

            // Gram-Schmidt orthogonalize
            Vector3.OrthoNormalize(ref n, ref t);

            tangents[i].x = t.x;
            tangents[i].y = t.y;
            tangents[i].z = t.z;

            // Calculate handedness
            tangents[i].w = (Vector3.Dot(Vector3.Cross(n, t), tan2[i]) < 0.0f) ? -1.0f : 1.0f;

        }

        theMesh.tangents = tangents;

    }

}